1. Field of the Invention
This invention generally relates to position determination, and more specifically to a system and method for position determination by impulse radio.
2. Background Art
In recent years, modern communications technology has provided various systems for position determination. The global positioning system (GPS) operated by the United States Department of Defense, for example, is a highly complex system of determining the position of an object. The GPS system depends on measuring the time-of-flight of microwave signals from three or more orbiting satellite transmitters by a navigation receiver that computes the position of the mobile unit. According to the GPS system, each satellite broadcasts a time-stamped signal that includes the satellite""s ephemeris, i.e., its own position. When the mobile unit receives a GPS signal, the mobile unit measures the transmission delay relative to its own clock and determines the pseudo-range to the transmitting satellite""s position. The GPS system requires three satellites for two-dimensional positioning, and a fourth satellite for three-dimensional positioning.
Another approach is that employed by the U.S. Navy""s TRANSIT system. In that system, a mobile unit performs continuous doppler measurements of a signal broadcast by a low earth orbit (LEO) satellite. The measurements continue for several minutes. The system usually requires two passes of the satellite, necessitating a wait of more than 100 minutes. In addition, because the position calculations are performed by the mobile unit, the satellite must broadcast information regarding its position, i.e., its ephemeris. Although the TRANSIT system is capable of high accuracy (on the order of one meter), the delay required is unacceptable for commercial applications.
Although these systems accurately determine the unknown position of an object, they are extremely complex, and, more importantly, expensive to implement. For example, both the GPS and TRANSIT systems require multiple satellites, sophisticated receivers and antennas that require hundreds of millions dollars of investments. Also, response times of GPS and TRANSIT systems are typically slow due to their narrow bandwidth. Furthermore, since these systems depend on orbiting satellites, they require an unimpeded view of the sky to operate effectively.
There is a great need in many different fields for a simple, less expensive alternative to complicated position determination systems. One such area is a typical shipping terminal, e.g., a major sea-port or an airport. In a sea-port, containers having valuable cargo are stored at warehouses or are left in designated places in the terminals. Also, containers are sometimes moved from one section of the port to another section in preparation for their eventual loading into a cargo ship or being picked up by trucks or railcars after being unloaded from a cargo ship. Often it is necessary to determine the location of one or more containers. However, it is difficult to identify one or more containers among hundreds, or thousands of containers in a terminal. Similar problems are also encountered in airports and railway terminals where containers are kept in storage sites.
A simple, less expensive position determination system is also desirable for locating police units. Such a position determination system can be used as a vehicle locator system. A city dispatcher would be able to quickly and efficiently dispatch police units if the dispatcher has pre-existing knowledge of each unit""s locations. Currently city dispatchers use mobile phones to communicate with police units in order to know their locations. However, using mobile phones to determine the positions of the police units has some disadvantages. Use of mobile phones is expensive and time consuming. Also, when a police officer is not in the car, it is not possible to determine the unit""s location.
Recently, the FCC has mandated that all cell phone systems implement position determination for use in emergency call location. In addition, there is a need for position determination as part of cell phone security, fraudulent use, and zone handoff algorithms. These requirements are difficult to meet and GPS is not adequate to reliably deliver the required accuracy.
For these reasons, it is clear that there is a need for a simple, low cost position determination system.
The present invention is directed to a system and a method for position determination using impulse radios. According to one embodiment of the present invention, a first transceiver having a first clock providing a first reference signal is positioned. A second transceiver whose position is to be determined is spatially displaced from the first transceiver. The second transceiver has a second clock that provides a second reference signal.
To determine the position of the second transceiver, a first sequence of pulses are transmitted from the first transceiver. The first sequence of pulses are then received at the second transceiver and the second transceiver is then synchronized with the first sequence of pulses. Then, a second sequence of pulses are transmitted from the second transceiver. The first transceiver receives the second sequence of pulses and the first transceiver is synchronized with the second sequence of pulses. A delayed first reference signal is generated in response to the synchronization with the second sequence of pulses. Then, a time difference between the delayed first reference signal and the first reference signal is measured. The time difference indicates a total time of flight of the first and second sequence of pulses.
Then, the distance between the first and the second transceiver is determined from the time difference. Then, the direction of the second transceiver from the first transceiver is determined using a directional antenna. Finally, the position of the second transceiver is determined using the distance and the direction.
In another embodiment of the present invention a plurality of first transceivers and a second transceiver are placed such that each transceiver is spaced from the others. The distance between each first transceiver and the second transceiver is measured. Then, the position of the second transceiver is determined using a triangulation method.
In yet another embodiment of the present invention, the second transceiver is placed in a mobile telephone whose position is to be determined. This allows a user of a mobile telephone to determine his or her exact location.
The position determination system according to the present invention provides numerous advantages over conventional position determination systems described before. For example, the present invention does not require the use of expensive orbiting satellites. Thus, the present invention is less expensive to implement. Also, signals from orbiting satellites are often impeded by obstacles, such as trees or overhead structures. Since, the present invention does not require the use of orbiting satellites, the operation of the present invention is not impeded by obstacles, such as trees or other structures. Furthermore, since the present invention utilizes ultra-wideband signals, it provides a relatively fast response time. As a result, the position of an object can be determined much faster than it would be possible using existing systems.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.